A transparent conductive film, because of having high conductivity and high transmittance in a visible light region, has been utilized in an electrode or the like, for a flat panel display or a solar cell and other various light receiving elements, as well as a heat ray reflection film for an automotive window or construction use, an antistatic film, and a transparent heat generator for various anti-fogging for a refrigerator showcase and the like.
As a practical transparent conductive film, there has been included a thin film of tin oxide (SnO2)-type, zinc oxide (ZnO)-type, indium oxide (In2O3)-type. As the tin oxide-type, the one containing antimony as a dopant (ATO), or the one containing fluorine as a dopant (FTO) has been well known, and as the zinc oxide-type, the one containing aluminum as a dopant (AZO), or the one containing gallium as a dopant (GZO) has been well known. However, the transparent conductive film most widely used industrially is the indium oxide-type. Among them, indium oxide containing tin as a dopant is called an ITO (Indium-Tin-Oxide) film, and has been utilized widely, because, in particular, a film with low resistance can be obtained easily.
Many of the transparent conductive films are n-type degenerated semiconductors, and electrons of carriers largely contribute to enhance electrical conductivity. Therefore, conventionally, in order to make low resistance of the ITO film, carrier electron concentration has been made to increase as high as possible.
The ITO film has been known to have a crystallization temperature of generally about 190 to 200° C., and bordering on this temperature, an amorphous film is formed at a low temperature side, or a crystalline film is formed at a high temperature side. For example, in the case of film-formation by a sputtering method while maintaining a substrate at room temperature, the amorphous film is obtained, because thermal energy required in crystallization cannot be given. However, in the case where a substrate temperature is crystallization temperature or higher, for example, about 300° C., the crystalline film is formed.
In the amorphous film and the crystalline film of ITO, generation mechanism of carrier electrons is different. In general, in the case of the amorphous ITO, nearly all of the carrier electrons are generated by oxygen deficiency. On the other hand, in the case of the crystalline ITO, generation of the carrier electrons is expected by not only oxygen deficiency but also tin dopant. Indium oxide takes a crystal structure called bixbyite of a stable cubic system crystal phase, under normal pressure or pressure lower than that. By substitution of a lattice point of tri-valent indium in the bixbyite structure with tetra-valent tin, the carrier electrons are generated. Tin is an element which is capable of increasing carrier electron concentration most, as a dopant, and it has been known that the addition of 10% by weight as converted to tin oxide is capable of decreasing resistance most. That is, by converting the ITO film to a crystalline film, carrier electrons are generated in a large quantity by both of oxygen deficiency and the tin dopant, and therefore it is possible to form a film exhibiting lower electric resistance as compared with an amorphous film having only oxygen deficiency.
However, in recent years, with diversification of electronics devices, such a transparent conductive film has been required which shows refractive index higher than that of the ITO film, and low electric resistance equivalent to that of the ITO film. A typical application of such a transparent conductive film includes a blue LED or a solar cell. As a light emitting layer of the blue LED, a gallium nitride layer is used, and an optical characteristics of this gallium nitride layer have a refractive index as high as about 2.4. In order to enhance efficiency of light extraction efficiency from the light emitting layer, it is necessary to enhance consistency of refractive indexes of the transparent conductive film and the gallium nitride layer, and the transparent conductive film is required to have a refractive index of as near as 2.4. Refractive index is a value specific to a substance, and generally known refractive index of indium oxide is as low as 1.9 to 2.0. In addition, the transparent conductive film is required to have low surface resistance. It is because current diffusion is not sufficient in a film surface direction, as electrical characteristics of the gallium nitride layer. However, when it is tried to decrease electric resistance by increasing carrier electron concentration, refractive index of the indium oxide-based transparent conductive film becomes lowered further than 1.9. As described above, because the ITO film is a material having significantly increased carrier (electron) concentration owing to tin as a dopant, trying to obtain a crystalline film with such a low resistance results in decreasing refractive index, and this has been a problem to be solved.
In addition, in the blue LED, other than refractive index or specific resistance, in patterning property or the like by wet etching, characteristics superior than that of the ITO film is required. Therefore, such a production process is preferable that makes low resistance by performing patterning by wet etching using a weak acid on the amorphous transparent conductive film formed at low temperature, and then by heat treatment under non-oxidative atmosphere to crystallize the film. By using this process, it is possible to form a transparent electrode having highly fine patterning.
As applications where the transparent conductive film is required to have superior characteristics over the ITO film, there is a solar cell. In the case where the transparent conductive film having high transmittance of not only visible light but also infrared light is used as a surface electrode of a solar cell, solar light can be captured efficiently. However, in case of ITO film, it is capable of decreasing specific resistance, but because of high carrier electron concentration, there was a problem of high reflectance or absorption of infrared light, and thus decreasing transmittance.
In addition, in the case of using it as a part of a rear surface electrode, there may be the case of using a transparent conductive film having enhanced refractive index, for enhancing incorporation efficiency of solar light and performing the adjustment of refractive index of the whole module. Also in this case, the ITO film was insufficient because of the same reason as in a blue LED application. However, in a solar cell application, because low specific resistance is considered as important, it is not required such high precision patterning by wet etching using a weak acid, that is required in the blue LED.
As one method for enhancing refractive index of the indium oxide-based transparent conductive film, there is a method for adding an oxide having high refractive index.
For example, in PATENT LITERATURE 1, there has been proposed a sputtering target, which is capable of efficiently film-forming a transparent thin film with superior moisture barrier property, and gives little damage to the above silver-based thin film during this film-formation. In this LITERATURE, there has been described a sputtering target composed of an electric conductive transparent metal oxide containing an oxide of a metal element substantially not having a solid solution region with silver, wherein content ratio of the above metal element, substantially not having a solid solution region with silver, is 5 to 40% by atom relative to the metal element of the electric conductive transparent metal oxide. Preferably, it has been described at least a titanium element or a cerium element, as the metal element substantially not having a solid solution region with silver, and indium oxide, as the electric conductive transparent metal oxide.
Further, in the PATENT LITERATURE 1, there has been described that because the metal oxide of the titanium element or the cerium element, of the metal element substantially not having a solid solution region with silver, has a high refractive index of 2.3 or higher, and as said high refractive index material, total content rate of the titanium element and the cerium element is 5 to 40% by atom to the metal element of the electric conductive transparent metal oxide, it is possible to increase refractive index of the transparent conductive film, film-formed by using this sputtering target, up to about 2.1 to 2.3.
In addition, in PATENT LITERATURE 2, there has been proposed a sputtering target of a sintered body of a mixed oxide, having contained tin oxide and/or titanium oxide each in an amount lower than mixing ratio of the substrate, into a mixed oxide having indium oxide and cerium oxide as the substrate, in film-forming a transparent thin film of an conductive film with a composition sandwiching the silver-type thin film, so as to be able to efficiently perform film-formation of the transparent thin film with superior moisture barrier property, and also to provide a sputtering target where the above silver-based thin film little receives damage in this film-formation.
Also in this LITERATURE, similarly as in PATENT LITERATURE 1, there has been described that, because cerium oxide has high refractive index, also refractive index of the mixed oxide of indium oxide and cerium oxide becomes high, accompanying with content ratio of cerium oxide. Further, because, in the mixed oxide of indium oxide and cerium oxide, cerium oxide does not have sufficient electrical conductivity, electrical conductivity of a target of the mixed oxide abruptly decreases accompanying with increase in mixing ratio of cerium oxide, and thus providing a target with low electrical conductivity, which makes direct-current sputtering difficult.
In addition, in PATENT LITERATURE 3, there has been proposed a transparent conductive film having a three-layer structure, where a silver-based thin film having a thickness of 5 to 20 nm is sandwiched by transparent oxide thin films, as a transparent conductive film having high electrical conductivity and visible light transmission, as well as superior storage stability due to little time degradation. In this LITERATURE, there has been described a transparent conductive film where the above transparent oxide thin film is a mixed oxide of a first substrate containing one or more kinds of metal oxides, which easily make a solid solution state with silver, and a second substrate containing one or more kinds of metal oxides, which are difficult to make a solid solution state with silver; and the silver-based thin film is a transparent conductive film, which is a silver alloy containing at least gold, preferably the first substrate is indium oxide and the second substrate is cerium oxide.
In the above PATENT LITERATUREs 1 and 2, there has been described that a film-formation temperature is desirably set at 180° C. or lower or room temperature, and also in Example, the film-formation temperature is 180° C. or lower, and heating treatment after film-formation is performed at 220° C. at the highest, that is, it is heated at lower temperature than crystallization temperature of the transparent conductive film having a composition shown in the Example.
Any of the transparent conductive film with high refractive index, disclosed in these PATENT LITERATUREs 1 to 3, is an amorphous film. To begin with, because the PATENT LITERATUREs 1 and 2 are proposals relating to the transparent conductive film superior in moisture barrier property, the transparent conductive film should be amorphous, similarly as a moisture barrier thin SiO2 film included in the PATENT LITERATURE 1. It is because, in case of a crystalline film, moisture invades via crystal grain boundary, causing damage of the silver thin film. In addition, in the PATENT LITERATURE 3, there has been described that performing of annealing treatment at a temperature of 200° C. or higher is capable of increasing electrical conductivity of the whole three-layer film. However, the object of this annealing treatment is to make low resistance of the silver-based thin film composing a three-layered film, and not to make the transparent thin film crystalline. In a case of forcedly performing of heating treatment at such a high temperature, for example, over 300° C., to make the transparent thin film crystalline, the silver thin film will receive damage also by thermal oxidation.
As described above, the transparent thin films with high refractive index, disclosed in the PATENT LITERATUREs 1 to 3, are only amorphous films, and there has been no disclosure on crystalline transparent conductive films.
Moreover, the PATENT LITERATURE 1 has disclosed electric resistance of a thin film composed of a three-layered structure of a transparent thin film/a silver thin film/a transparent thin film, however, there has not disclosed electric resistance of the transparent thin film alone.
In the PATENT LITERATURE 1, preferable composition range is a cerium element of 10% by atom to an indium element, and it has been confirmed that formation of an amorphous transparent conductive film with this composition at room temperature, so as to have a film thickness of 200 nm, provides a surface resistance of 100 Ω per square, and it is 2.0×10−3 Ω·cm as specific resistance. A transparent electrode for a blue LED requires a low specific resistance of at least 8.0×10−4 Ω·cm or lower, however, the amorphous film of the PATENT LITERATURE 1 is difficult to be applied thereto due to having high electrical resistance as above.
On the other hand, PATENT LITERATURE 4 has proposed an amorphous transparent electric conductive thin film having superior smoothness and high work function, an oxide sintered body, which is capable of stable film-formation of said transparent electric conductive thin film, and a sputtering target using the same. And, there has been described that said oxide sintered body desirably contains cerium in 3% by mass to 20% by mass, tin in 0.1% by mass to 4% by mass, and titanium in 0.1% by mass to 0.6% by mass, the remaining being substantially composed of indium and oxygen; and still more, cerium, tin and titanium are in a solid solution state in an indium site; sintered density is 7.0 g/cm3 or higher, and average crystal particle diameter is 3 μm or smaller.
In this PATENT LITERATURE 4, only an amorphous film has been shown, and there has no investigation on formation of a crystalline transparent conductive film using the above sputtering target, or enhancing refractive index thereby. In addition, the oxide sintered body of the PATENT LITERATURE 4 contains tin, however, there has not been referred to adverse effects of tin on decrease in refractive index.
In addition, PATENT LITERATURE 7 has proposed a transparent conductive film only containing indium oxide and cerium oxide, without containing tin or titanium as above. The PATENT LITERATURE 7 has proposed a sputtering target characterized in that, in the case of observing crystal peaks by X-ray diffraction, presence of peaks derived from indium oxide and cerium oxide are observed, and in the case of performing EPMA measurement, diameter of a cerium oxide particle dispersed in indium oxide is measured to be 5 μm or smaller, and still more characterized in that, in the sputtering target composed of indium oxide and cerium oxide, Ce/(In+Ce) is 0.005 to 0.15, and has described film-formation of the transparent conductive film by a sputtering method using this sputtering target. That is, the transparent conductive film of the PATENT LITERATURE 7 contains cerium in 0.005 to 0.15 as Ce/(In+Ce).
In the PATENT LITERATURE 7, there has not been referred to surface roughness of the transparent conductive film at all, however, there was a problem of increased light scattering in an application such as a blue LED or a solar cell, caused by loosing surface smoothness and giving rough surface, because of film-formation of the transparent conductive film by a sputtering method.
In addition, in the PATENT LITERATURE 7, there has been described a production of the transparent conductive film by a production method containing a step for film-formation of the transparent conductive film by a sputtering method using the above sputtering target, and a step for crystallization by heating the transparent conductive film, thus film-formed, at a temperature range of 200° C. to 250° C. However, in the case of containing indium oxide as a main component, and cerium, because of increased crystallization temperature of the transparent conductive film, a sufficiently progressed crystallization state cannot be attained only by heating in a relatively low temperature range of 200° C. to 250° C., although crystallization may happen in some cases. For sufficient progress of crystallization, heating is necessary in a temperature range at least over crystallization temperature by 50° C., that is, in a temperature range over 250° C. The case where crystallization progressed sufficiently provides the transparent conductive film having high mobility of carrier electrons, useful in an application of a blue LED or a solar cell or the like. However, such a film as described in the PATENT LITERATURE 7 has not been possible to attain sufficient characteristics as the transparent conductive film, when considered the application of a blue LED or a solar cell or the like, because of no sufficient progress of crystallization by heating in a low temperature range, and thus no enhancement of carrier mobility.
On the other hand, as another method for obtaining the transparent conductive film with enhanced refractive index, there is also a method for selecting a material exhibiting higher refractive index than that of indium oxide.
For example, PATENT LITERATURE 5 has proposed a transparent metal material composed of a transparent and electric conductive material, which is able to supply stably and is superior in chemical resistance or the like, along with a transparent electrode. In this LITERATURE, there has been described that low resistivity can be expressed while maintaining inner transmittance, by forming an metal oxide layer composed of an anatase-type crystal structure, and composing the metal oxide layer with M:TiO2; as well as electrical conductivity can be extremely enhanced, while maintaining transparency degree, by preparing M:TiO2 obtained by substituting the Ti site of the anatase-type TiO2 with other atoms (Nb, Ta, Mo, As, Sb, W and the like).
Because refractive index of the anatase-type TiO2 is about 2.4, the material of the PATENT LITERATURE 5 is optimal in matching the consistency with refractive index with a gallium nitride layer of the afore-mentioned blue LED. However, sufficient characteristics as a transparent electrode for the blue LED have not been obtained, because of having higher specific resistance by one order as compared with the ITO film. In addition, there is also a problem of slower film-formation rate, as compared with the ITO film, resulting in lower production efficiency.